1. Field of the Invention
The present invention relates to an array substrate, a method of inspecting an array substrate, and a liquid crystal display (hereinafter also referred to as “LCD”).
2. Description of Related Art
A liquid crystal display is generally light and thin, and requires less electric power. Therefore, liquid crystal displays are widely used as display elements in television receivers, hand-held terminal, graphic displays, etc. Especially, attention is drawn to active matrix type liquid crystal displays using thin film transistors (hereinafter also referred to as “TFTs”) as switching elements. Since an active matrix type liquid crystal display is superior in its quick responsiveness, and picture elements can be more densely arranged in an active matrix type liquid crystal display, it is expected that with such a type of display, image quality of the display screen may be improved, the size of the display screen may be enlarged, and colorization of the display can be achieved.
As shown in FIG. 10, an active matrix type liquid crystal display typically has an array substrate 100, an opposite substrate 200, and liquid crystal (not shown) disposed therebetween. The array substrate 100 includes a transparent insulating substrate (e.g., a glass substrate) 101 having a display area 102a. A plurality of signal lines 103 and a plurality of scanning lines 104 are arranged in a matrix form on the display area 102a. Switching elements 105 formed of thin film transistors and pixel electrodes 106 each corresponding to one of the switching elements are provided at the intersections of the signal lines 103 and the scanning lines 104. A gate of each switching element 105 is connected to the corresponding scanning line 104. One of a source and a drain of each switching element is connected to the corresponding signal line 103, and the other is connected to the corresponding pixel electrode 106.
The array substrate 100 may further include a driving circuit 110 having TFTs and external terminals 120, connected to the driving circuit 110, for receiving electrical power or signals from external devices on a non-display area 102b which is the area on the periphery of the transparent insulating substrate 101.
The opposite substrate 200 includes a transparent insulating substrate 201 and a common electrode 203 formed thereon as a transparent conductive film of ITO (Indium Tin Oxide).
The above-mentioned substrates 100 and 200 are placed so as to be opposite to each other, with a predetermined space being left therebetween. A sealing material 300, which is applied to the non-display area 102b so as to surround the display area 102a of the array substrate 100, sticks the substrates together. As shown in FIG. 10, the portion applied the sealing material 300 has an inlet 301 for inserting a liquid crystal material therethrough. After the above-mentioned substrates 100 and 200 are stuck together, a liquid crystal composite (not shown) is fed into the space between the substrates through the inlet 301. Consequently, the substrates are sealed to obtain a liquid crystal display. In the case of a color liquid crystal display, a color filter layer is added to either of the opposite substrate 200 and the array substrate 100.
FIG. 11 shows the structure of an array substrate of a conventional liquid crystal display. The array substrate shown in FIG. 11 includes a transparent substrate, a plurality of scanning lines 11 for use in scanning operations, and a plurality of signal lines 12 for transmitting picture signals, the scanning lines 11 and the signal lines 12 being arranged in a matrix form on one surface of the transparent substrate. Thin film transistors 13 as switching elements are provided at the intersections of the scanning lines 11 and the signal lines 12. A gate of each thin film transistor is connected to the corresponding scanning line 11, a source thereof is connected to the corresponding signal line 12, and a drain thereof is connected to a pixel electrode 14. One end of a storage capacitor 15 is connected to each pixel electrode 14, and the other end thereof is connected to a storage capacitor line (hereinafter also referred to as “Cs line”) 16.
Each scanning line 11 is connected to a scanning line driving circuit 18 provided at the left side of the array substrate 100 in FIG. 11. Each signal line 12 is connected to a signal line driving circuit 19 provided in the lower side of the array substrate 100 in FIG. 11. The storage capacitors 16 are commonly connected, via a line 31, to an OLB (outer lead bonding) pad 26 in an OLB pad group 20 formed as an input/output terminal group on the bottom end of the substrate in FIG. 11.
A voltage with the horizontal scanning cycle is applied from the scanning line driving circuit 18 to each scanning line 11, from the top to the bottom. A voltage corresponding to a picture signal is applied to each signal line 12 from a video bus serving as a picture signal supplying line included in the signal line driving circuit 19. Accordingly, a thin film transistor 13 is turned on at the timing a selecting signal is applied from the scanning line 11 to sample a voltage corresponding to the picture signal sent from the signal line 12, and to apply the voltage to the pixel electrode 14. Thus, the difference between the voltage applied to the pixel electrode 14 and the voltage applied to the common electrode (not shown) is applied to the liquid crystal layer. Accordingly, the liquid crystal layer is driven by the electric field caused, thereby to perform a display operation.
In the conventional liquid crystal display shown in FIG. 11, the scanning line driving circuit 18 and the signal line driving circuit 19 are formed on the array substrate 100, and a semiconductor layer serving as an active layer of a thin film transistors 13 is formed of polycrystalline silicon. Such a display is called p-Si type TFT-LCD. If the semiconductor layer of the thin film transistor is formed of amorphous silicon, such a display is called a-Si type TFT-LCD. A TFT having a semiconductor layer formed of amorphous silicon cannot be used in a driving circuit since the mobility thereof is not so good as compared with a TFT having a semiconductor layer formed of polycrystalline silicon. Therefore, it is difficult to provide a driving circuit on the array substrate 100 of an a-Si type TFT-LCD. Accordingly, the array substrate of an a-Si type TFT-LCD is formed only of pixel parts, and no driving circuit is housed therein, as shown in FIG. 12. A driving circuit for this type of display is formed, separate from the array substrate 100, as a semiconductor integrated circuit (driver IC), and connected to OLB pads 211-21n and 221-22n of the array substrate 100 by the TAB (tape automated bonding) method, etc.
One of the differences between the p-Si type TFT-LCD shown in FIG. 11 and the a-Si type TFT-LCD shown in FIG. 12 lies in the OLB pads. In the a-Si type TFT-LCD, the signal lines and the scanning lines are directly connected to the OLB pads. Therefore, the number and the pitch of the OLB pads are the same as those of the signal lines and the scanning lines. In the p-Si type TFT-LCD, the signal lines and the scanning lines are not directly connected to the OLB pads since they are driven by the built-in driving circuits 18 and 19. Inputs from the OLB pads are equal to inputs from the built-in driving circuits 18 and 19. Generally, the number of the OLB pads is about one order less than that of the signal lines or the scanning lines. Accordingly, it is possible to widen the pitch of the OLB pads so as to secure the connection reliability. The above-mentioned differences in OLB pads are shown in Table 1. The p-Si type TFT-LCD has an advantage that the degree of accuracy of the prober may be decreased, and the cost of equipment investment may be reduced as compared to the a-Si type TFT-LCD. Table 1 shows the case of XGA (Extended Graphics Array) capable 10.4-inch LCD generally used for personal computers, in which the values shown are approximate values.
TABLE 1 Number of PadsPad Pitcha-Si type TFT-LCD4000 60 (μm)P-Si type TFT-LCD200160 (μm)
The above-mentioned differences in OLB pads have effects on the inspection during the formation of the array. In the case of manufacturing an a-Si type TFT-LCD, after signal lines are formed during the array formation process, an inspection for short circuit and disconnection (hereinafter also referred to as “OS inspection”) is performed. This inspection includes putting probes on an OLB pad connected to one end of the signal lines and a probing pad connected to the other end of the signal lines, applying a predetermined level of voltage therebetween, and measuring the current flowing at that time. If the signal lines are correctly formed, a predetermined value of current, which is determined by the value of the applied voltage and the resistance of the signal lines, can be observed. If the signal line is disconnected (i.e., the circuit is open), no current flows, whereby a disconnection failure can be detected. Further, if, at the time of carrying out the inspection, a voltage, having the value different from that of the voltage applied to the signal line, is applied to the scanning lines and the storage capacitor lines, when the signal lines are short-circuited with these lines, an abnormal current flows, whereby a short circuit failure can be detected.
On the other hand, no OS inspection is included in the process of forming an array of a p-Si TFT-LCD. The reason for this is that no OLB pad for putting the probe thereon is provided at the end of the signal lines.
As mentioned previously, in the process of forming an array of a p-Si type TFT-LCD, no OS inspection is performed. Although an array test is performed at the last step of the array forming process, the rate of detecting signal line failure is not so good since in a p-Si type TFT-LCD, the picture element portion is inspected via built-in driving circuits, which causes the deterioration of the S/N (signal to noise) ratio. Thus, the rate of detecting signal line disconnection, short circuit, etc. of a p-Si type TFT-LCD is lower than that of an a-Si type TFT-LCD. As a result, the number of defective arrays sent to the cell process becomes rather large, thereby incurring unnecessary manufacturing costs.
Furthermore, if an OS inspection is intended to be performed for a p-Si type TFT-LCD, it is necessary to provide probing pads for putting probes thereon at both ends of the signal lines. The size of a probing pad has to be about the same as that of an OLB pad. Providing such probing pads between the signal lines 12 and the signal line driving circuit 19 would cause the increase in size of the area of the signal line driving circuit 19. As the result, the compactibility of the liquid crystal module to be manufactured would be deteriorated.
Moreover, even if probing pads are provided, the pitch thereof should be the same as that of the signal lines. Accordingly, it is necessary to use, in an inspection of array substrate of a p-Si type TFT-LCD, a high accuracy prober intended for use in array substrate inspection of a-Si type TFT-LCD. This would increase the equipment investment cost.